Glucose stores the sun's energy in chemical form.
Adenosine triphosphate - ATP the molecule organisms can actually use for energy.
Respiration the breaking down of pyruvic acid using molecular oxygen.
Glycolysis

Splits the 6-carbon glucose molecule into two 3-carbon pyruvic acid molecules and 2 ATP molecules. This process is anaerobic, not requiring the presence of molecular oxygen.

Fermentation - the breaking down of pyruvic acid without using molecular oxygen.

At this point, there are three possibilities:

Aerobic respiration

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The Krebs Cycle, also known as the Citric Acid Cycle, produces 2 ATP molecules, 10 carrier molecules, and CO2 from each glucose molecule.

The Electron Transport Chain then produces 34 ATP molecules and H2O from the carrier molecules.

The bulk of a cell's ATP must be produced in the mitochondria.

To be energy efficient, particles must move through the membrane with little energy expense.

Electron transport produces an uneven charge on the mitochondria membrane.

This allows protons to move through the membrane by electrical charge attraction.

Protons now move through the membrane and are used by ATP synthase enzyme to make ATP.

Lactic acid fermentation

- Occurs in animal cells due to a lack of oxygen, causing muscle soreness as lactic acid builds up. While no ATP is produced, a carrier compound is produced, allowing glycolysis to continue.

Alcoholic fermentation

- Occurs in some plants and unicellular organisms.
The process converts pyruvic acid into ethyl alcohol and a carrier compound, which allows glycolysis to continue.